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Store::getFSAccessor: Do not include the store dir

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Rather than "mounting" the store inside an empty virtual filesystem,
just return the store as a virtual filesystem. This is more modular.

(FWIW, it also supports two long term hopes of mind:

1. More capability-based Nix language mode. I dream of a "super pure
   eval" where you can only use relative path literals (See #8738), and
   any `fetchTree`-fetched stuff + the store are all disjoint (none is
   mounted in another) file systems.

2. Windows, where the store dir may include drive letters, etc., and is
   thus unsuitable to be the prefix of any `CanonPath`s.

)

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
This commit is contained in:
John Ericson 2025-02-19 18:46:37 -05:00
parent 2455bda91b
commit eb643d034f
18 changed files with 146 additions and 50 deletions
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20
src/libexpr/eval.cc
View file

@ -264,7 +264,25 @@ EvalState::EvalState(
auto storeFS = makeMountedSourceAccessor(
{
{CanonPath::root, makeEmptySourceAccessor()},
{CanonPath(store->storeDir), makeFSSourceAccessor(realStoreDir)}
/* In the pure eval case, we can simply require
valid paths. However, in the *impure* eval
case this gets in the way of the union
mechanism, because an invalid access in the
upper layer will *not* be caught by the union
source accessor, but instead abort the entire
lookup.
This happens when the store dir in the
ambient file system has a path (e.g. because
another Nix store there), but the relocated
store does not.
TODO make the various source accessors doing
access control all throw the same type of
exception, and make union source accessor
catch it, so we don't need to do this hack.
*/
{CanonPath(store->storeDir), store->getFSAccessor(settings.pureEval)},
});
accessor = settings.pureEval
? storeFS